The present invention relates to a vibrato signal generating arrangement for an electronic musical instrument.
An electronic musical instrument is generally accompanied with a vibrato device. In the electronic musical instrument, it is more preferable to add vibrato after the lapse of a relatively short time from key depression and then increase such vibrato gradually than to add vibrato immediately after key depression. The former vibrato is known as the delayed vibrato. In such delayed vibrato, a clear tone pitch of the depressed key may be obtained at time of key depression.
A typical example of existing delayed vibrato devices is provided with a series combination of a resistor and a capacitor connected between a DC power supply and ground, an astable multivibrator for generating vibrato signals connected in parallel with the capacitor with one end connected to ground, a one-shot multivibrator triggered by a trigger pulse obtained at key depression to generate an output pulse with a predetermined duration, and a switch circuit connected in parallel with the capacitor and allowed to conduct by the output pulse from the one-shot multivibrator.
According to the aforesaid vibrato device, the capacitor discharges for the duration (about 10ms) of the output pulse from the one-shot multivibrator, and thus the astable multivibrator generates no vibrato signal. After extinguishment of the output pulse from the one-shot multivibrator, the capacitor charges gradually. On start of charging of the capacitor, the astable multivibrator starts to oscillate and generate an output signal with an amplitude depending on the voltage across the capacitor. Namely, the output amplitude of the astable multivibrator increases gradually.
The above-mentioned vibrato device has a somewhat complicated construction due to the provision of the one-shot multivibrator. The output amplitude of the astable multivibrator fluctuates between the supply voltage and the ground potential. Therefore, the DC level at the output of the astable multivibrator fluctuates while the output amplitude of the astable multivibrator varies. Thus, even though the astable multivibrator is coupled to a tone generator through a coupling capacitor, the average pitch of a tone generated will vary due to the fluctuation of the DC level.
Another example of delayed vibrato devices is proposed in the U.S. Patent Application Ser. No. 787,149 filed Apr. 13, 1977 and assigned to the same assignee as the present application. The vibrato signal generating arrangement proposed in the above-mentioned application comprises envelope signal generating means, a unidirectional clipping device for clipping the envelope signal, and vibrato signal generating means driven by the output of the unidirectional clipping device to generate a vibrato signal. With this arrangement, the time delay for delayed vibrato is determined virtually by the cut-in voltage of the unidirectional device or diode which varies with temperature variation. This arrangement has, therefore, a drawback that the time delay for delayed vibrato varies with temperature variation.